A boundary microphone is used by being placed on a table or a floor, for example, at a TV studio or a conference hall, and therefore it is called a surface mount microphone. In order to make the presence of boundary microphone inconspicuous, the boundary microphone uses a flat microphone case having a thin appearance of restrained height (for example, refer to Japanese Utility Model Application Publication No. H07-43015).
Specifically, the microphone case of boundary microphone has a flat base portion whose upper surface side is open, the base portion being mounted with a microphone unit and a circuit board, and a microphone cover having a large number of openings (acoustic wave introduction holes) is put from the upside of the base portion. Usually, the base portion is formed by casting such as zinc die casting, and as the microphone cover, a punching plate or a wire netting body of hard metal wire is used.
Since the boundary microphone is used by being placed on a table or a floor, water (for example, drinking water spilled by a toppled cup) is often poured on the boundary microphone or the boundary microphone is often dipped in water, as compared with a stand microphone and a hand-held microphone.
Since water causes malfunction of microphone, for the boundary microphone, the microphone cover is subjected to water repellent processing, or a gasket such as sponge having water repellency is inserted in a connecting portion between the microphone cover and the base portion. However, since the microphone cover is flat and wide in area, and moreover is formed with a large number of openings, it cannot be said that only the water repellent processing provides a sufficient water proofing effect.
The boundary microphone has been used indoors exclusively. Recently, however, the inconspicuous presence of boundary microphone has been appreciated, and the use thereof in the field of security has been studied. As one example thereof, the boundary microphone is used jointly with a security video camera installed out of doors. This joint use intends to further enhance the reliability of security by picking up not only image but also sound out of doors. In the case where the boundary microphone is installed out of doors, stronger waterproof is required than in the case where the boundary microphone is used indoors. In addition, countermeasures against wind noise must be taken.